The trend in recent years and projected rise in numbers to 2020
By Gary Eason
Education editor, BBC News website
The squeeze on infant and primary school places that has begun in some parts of England looks set to get worse.
Population trend data show a decade of decline in the number of pupils in the younger years has ended.
Statisticians now expect the total to increase over the next decade by more than half a million.
This is offset to an extent by the existence of about half a million surplus primary school places in England - 12% of total capacity.
The problem is these surpluses are not necessarily in the areas where the pressure for more places will be felt most.
Local authorities' forecasts show that by 2013 the number of primary pupils is set to increase by some 4%.
More than 2,300 primary schools (14% of the total) had more than a quarter of their places unfilled last year.
Educationist Professor Alan Smithers of Buckingham University said: "The government has missed a trick in not taking the opportunity of the falling numbers of primary age pupils to improve the pupil-teacher ratio.
"It has always struck me as odd that the government has been content with a much higher ratio in the primary than the secondary phase since younger pupils require much more individual attention."
He said he challenged the political parties to say whether they would yet make use of the opportunity of the dip in pupil numbers.
"Further growth in numbers is just resuming so there is still time to intervene."
In England's secondary schools the dip in the birth rate that has now worked its way through the primary sector will continue.
There are various reasons why the birth rate in each area changes
This means the number of surplus secondary school places will continue to grow, from about 299,000 last year to a projected 376,000 by 2015 (11% of total capacity).
"There are big differences between local authority areas," says the guidance for education planners on the Teachernet website, produced by England's Department for Children, Schools and Families.
In 2008 the highest percentage of surplus places in any authority was 24% - in Knowsley, in Merseyside.
Four other areas have at least a fifth of their places unfilled on average: Bristol, North East Lincolnshire, Nottingham and Southampton.
"There are also significant variations within local authorities. Many authorities have areas that buck the overall trend of declining pupil numbers across the authority as a whole.
"At the same time, pockets of surplus capacity can exist within areas of rising rolls, often as a result of parents choosing not to send their children to a particular school or schools."
Birth rate and migration
Officials give two main reasons for the falling rolls.
One is the birth rate, though the statistics can derive from various factors: families having fewer children, women having children later in life, or changes within an area meaning there are fewer women of childbearing age.
Individual mothers may be having more babies - but if there are fewer mothers in an area the number of children will still be falling.
Then there is migration, with some urban areas having seen many people leave but also some rural areas losing people as more outsiders buy second homes.
"This trend is offset in some places by increased immigration from abroad," the guidance says.
"The nature of this immigration will affect the demand for particular school places - for example, Polish families are likely to be seeking places for their children in Catholic schools."
The population trends and their impact on school provision are similar across the United Kingdom.
In Scotland there were 681,573 pupils in publicly funded schools last year.
The latest projections suggest that numbers will decrease steadily to 658,000 in 2014, then rise until peaking at 675,000 in 2022 and, as in England, the upturn will feed through the primary years first from next year.
Almost everywhere, proposals to close or amalgamate schools in response to empty desks prompt fierce local opposition.